This invention provides a downhole milling tool apparatus for use in milling through hard substances found in underground wells, including but not limited to barite (barium sulfate) deposits.
When drilling or working on an oil and gas well, an effective way to work safely is to “kill” the well. In essence, this means having a column of drilling mud on top of the pressurized wellbore fluids to prevent them from escaping the well at the surface. Depending on the pressure the well is producing, a different density of fluid or “mud weight” is used, with a higher mud weight to negate the effects of a higher pressure well. Barite (also known as barium sulfate, BaSO4) is used to increase the mud weight, or “weight up”. However, in use, some of the barite settles out of the mud and leaves deposits on the casing. When production tubing is installed inside that casing everything is clean; over time, however, some of this barium sulfate leaches inside the production tubing through wellbore fluids. This is especially prevalent at the connections, and at high downhole temperatures it hardens to a scale buildup and is difficult to drill through.
The rate of penetration (ROP) decreases significantly when drilling barium sulfate. Current tools on the market to combat this issue are plagued with decreasing ROP's (rates of penetration) and premature wear. Oftentimes, crews need to trip out of the well in order to change worn bits/mills before going back into the well. This increases the time spent working on a well and therefore increases cost.
In coiled-tubing drilling and workover operations, drilling fluid or drilling mud under pressure is used as the motive force for drilling or milling tools. In all drilling and workover operations, drilling fluid is used for cooling and for carrying away cuttings, in suspension, up the annulus toward the wellbore. It is characteristic of barite that grinding it past the flaky, large-particle state into a powdery, small-particle state causes the drilling-fluid-and-barite suspension to become more cement-like and less easily flowed up the annulus. Therefore, barite deposits need to be effectively chipped or flaked off without powdering. The initial contact of a given carbide bit with a barite deposit is not likely to cause powdering, but the subsequent action of following carbide bits in a rotating tool might cause such powdering.
Also, as stated above, drilling or milling through barite or substances of similar character are very tough on carbide bits, highlighting a need to chip or flake, but not powder, with as little wear to the carbide bits as possible. The present state of the art does not provide for these needs.
There is accordingly a need for a milling tool that can increase the ROP, but also be durable enough to go through barite without issue.